The invention relates to NMR imaging methods. More specifically, the invention relates to three-dimensional NMR imaging methods in which selective excitation is used to controllably select a thick planar slab of excited nuclear spins in a larger NMR imaging sample, and in which the spatial information needed to construct a series of several tomographic section images of the thick slab is collected simultaneously.
Several NMR imaging methods have been developed for imaging relatively thin planar regions of an imaging sample. One of the earliest methods is the sequential point method in which spatial information is gathered from one point at a time during a point-by-point scan of the entire region of interest either mechanically or electronically. A somewhat more efficient and sophisticated imaging method is the sensitive line method in which spatial information is collected simultaneously from one entire line at a time during a line-by-line scan of the region of interest, rather than from a single point at a time. Planar imaging methods are even more efficient since data are collected from an entire plane simultaneously.
In order to produce images from a three dimensional object, however, it is more efficient to gather imaging information line-by-line than point-by-point; better still, from entire planes rather than line-by-line. In NMR imaging, as in all other forms of imaging, it is desirable to gather as much information as possible in a given time. Therefore, it is even more desirable to collect information simultaneously from throughout a three-dimensional region of an imaging sample, than from a plane-by-plane scan. Probes for sensing NMR signals are generally tuned coils which are inherently sensitive to volumes, and thus NMR imaging is well suited to the acquisition of data from a three-dimensional portion of a sample. In fact, in the point, line, and planar imaging methods, considerable effort is required to restrict the data gathering process to the lesser regions of interest.
Three-dimensional NMR imaging schemes have been proposed by Richard R. Ernst in U.S. Pat. No. 4,070,611, issued Jan. 24, 1978, P. C. Lauterbur and C-M Lai, IEEE Transactions of Nuclear Science, NS-27, pp. 1227-1231 (1980), "Zeugmatography by Reconstruction from Projections"; and by P. Mansfield in U.S. Pat. No. 4,165,479, issued Aug. 21, 1979. Each of these schemes implicity assumes that the NMR imaging sample is limited in extent and contained within the region over which the tuned receiver coil is sensitive. This assumption, however, has a number of drawbacks.
One of the drawbacks is that the NMR signals from the periphery of the region over which the tuned receiver coil is sensitive may have different amplitudes or phases than NMR signals from nuclear spins at the center of the sensitive region. This results in fade-off or phase artifacts at the edge of the images or in the edge scans.
Another drawback is that the extent of the region of receiver coil sensitivity along the long axis of the human body (when acting as the imaging sample) can only be determined by the shape and strength of the applied RF fields and the shape of the receiver coil. In practice, such factors are not very effective because the RF magnetic fields cannot be produced with sharp boundaries. In addition, the precision of the RF field shaping is limited by the small number of turns allowed in usable RF coils at NMR imaging frequencies (typically 5 MHz) for hydrogen (.sup.1 H) in a 0.12 Tesla static magnetic field. The number of coil turns is limited by the distributed coil inductance and capacitance which place a limit on the highest frequency at which the coil can be made to resonate.
As yet another drawback, the acquisition of the NMR signal from a large portion of the imaging sample may be too great for the dynamic range of the imaging system electronics. For example, in order to construct an NMR image, it is necessary to know the NMR signal strength in each pixel in the imaging volume, wherein a pixel (picture element) is one small fraction of the total imaging volume. If three-dimensional reconstruction is by multiple angle projection reconstruction (as it is in one known three-dimensional imaging method), then the data array would be cubic and would contain 128.times.128.times.128 pixels, for instance, if the desired tomographic section image has 128.times.128 pixels. In three-dimensional imaging by multiple angle projection reconstruction, the resolution is the same in all directions (typically 2 mm.times.2 mm), and spatial information to construct a single tomographic section image cannot be obtained unless 128.times.128 projections are obtained. This is necessitated by the known fact that in this imaging method, the data must be isotropic. It is impossible to do only 128.times.128.times.10 projections to obtain the necessary data to construct 10 tomographic section images. Thus, the signal strength ratio (dynamic range) of one pixel to the NMR signal from the entire volume is 1 to 128.times.128.times.128, or approximately 1 to 2.times.10.sup.6. Additionally, because the data array is cubic, the data collecting and reconstruction processes are substantially lengthened.
In contrast, the NMR imaging method of the present invention does not require isotropic data. Image resolution in each tomographic section may be 2 mm.times.2 mm in the transverse plane (i.e., orthogonal to the long axis of the imaging sample), but in the direction of the long axis can be 10 mm, which is equal to the thickness of the region imaged by each tomographic section. Compared to the resolution of the previously-described multiple angle projection reconstruction method, the resolution in the direction of the long axis is decreased by a factor of 5. This, however, decreases both the length of time necessary to gather data, as well as the length of time necessary to analyze the data to reconstruct an image. Moreover, three-dimensional spatial information is gathered only from a selected thick slab having a thickness equal to the number, n.sub.z, of desired tomographic section images. In this manner, if only n.sub.z section images are desired, then only 128.times.n.sub.z NMR signals are needed to obtain the necessary spatial information. If n.sub.z =10, only 1280 projections (NMR signals) are needed, as compared to 128.times.128 in multiple angle projection reconstruction. It is also apparent that the required dynamic range is reduced by a factor of 128/n.sub.z.